1. Field
The present invention relates to a transmission apparatus for computer network communication that includes a phase locking unit, and a switching controlling unit, and a method of switching a phase lock reference frequency signal in the transmission apparatus.
2. Description of the Related Art
The importance of computer network communication such as the Internet has dramatically increased, and a further increase in communication speed of the computer network communication has been sought-after. For example, the Internet includes a backbone as a main transmission line, and in general, a broadband optical communication technology called a wavelength division multiplexing (WDM) has been used for the backbone.
The WDM is a technology that enables large capacity two-way high speed data communication, by multiplexing channels by transmitting light having different wavelengths on a single optical fiber. At present, a WDM with a transmission speed of 10 Gb/s and four multiplexed channels each with a transmission speed of 2.5 Gb/s at a wavelength is the mainstream.
Not only the optical communication, but every computer network communication is carried out with signals of a predetermined frequency. Transmission apparatuses such as a relay apparatus and a terminal apparatus in a communication network have a circuit called a phase-locked loop (PLL) that converts a frequency of a received signal to output a properly synchronized frequency signal by causing a frequency of an output signal matched to a frequency of an input signal or a reference frequency.
If the predetermined frequency of the communication needs to be changed over for some reason, a clock cross-connect distribution apparatus as proposed in Japanese Patent Application Laid-open No. H11-355259, for example, may be employed. The proposed clock cross-connect distribution apparatus can reduce the downtime and enables speedy recovery at the clock switching of an information processing apparatus, which employs a duplexed clock in a cross-connected manner, by using a low-frequency clock to switch a cross-connect-clock between the information processing apparatuses, and increasing the frequency of the clock supplied to each processing unit of the information processing apparatus after the clock switching.
However, a further increase in transmission speed in recent years gradually makes the conventional technique represented by that disclosed in the Japanese Patent Application Laid-open No. H11-355259 incapable of reducing the downtime and recovering speedily. On the other hand, an increased transmission speed is sought-after for the WDM. For example, an optical communication technology called a dense wavelength division multiplexing (DWDM) allows still larger capacity two-way high speed data communication by increasing the number of wavelengths to be multiplexed and by narrowing the channel interval.
The optical communication technology such as the DWDM is internationally-standardized as OTU3 of Optical Transport Network (OTN, an optical transmission standard recommended by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) in 2000), which is an optical communication network standard, and able to realize a transmission speed of 40 Gb/s.
In the optical communication technology with the transmission speed of 40 Gb/s, the PLL of the transmission apparatus changes a PLL reference clock to a clock of an internal oscillator, when an optical signal input into the transmission apparatus is interrupted. However, because of the high speed of 40 Gb/s, even a little switching time is a long time relative to the transmission speed, and significantly increases the number of error bits in the optical signal. The same applies when the optical signal is input and recovered.